【作者】 齐晓霓；

【导师】 刘震炎；

【作者基本信息】 上海交通大学 ， 动力工程与工程热物理， 2008， 博士

【摘要】 针对填料冷却塔存在的某些缺陷,无填料喷雾冷却塔去除了冷却塔的填料部分,对配水部分进行了较大改进,使循环水在淋水区域形成足够小的水滴,在无填料的情况下能够进行充分的水气热质交换,从而达到循环水冷却的目的,对处理高温、高浊度、易结垢循环水有着重要意义。本文对无填料喷雾冷却塔中热水水滴在空气中的运动规律及其与空气间的传热传质问题建立合理的数学模型并进行以热力学第二定律为基础的　分析,用数值模拟结果指导实验研究和理论分析,结合对喷破碎技术对海水无填料冷却塔性能进行研究,根据理论和实验研究设计上喷式无填料冷却塔,从而形成一套较为系统的研究内容,为无填料冷却塔的设计及运行提供理论指导。本文主要研究内容及结论如下:(1)在对水滴受力及运动分析进行研究的基础上,推导并求解水滴在无填料冷却塔中的运动方程,得到计算喷雾冷却塔高度、喷头安装位置、塔高及喷头塔壁距离的方法,为无填料冷却塔的设计提供理论依据。根据水滴与气流的不同方式,分析水滴在无填料冷却塔中的运动过程,得到不同风速下可能存在的水滴最大半径和最小半径。研究上喷式无填料冷却塔中水滴当量直径及初速度对水滴上升高度的影响,发现在空气速度和水滴直径不变的情况下,水滴初速度越大,上升高度越高,水滴容易飞出。空气速度和水滴初速度不变的情况下,并非小水滴容易飞出,水滴过大也会造成上升高度过高,给除水器造成过大压力。通过研究水滴当量直径对水滴水平运动距离的影响,发现水滴直径越小,水平运动距离越小,因此从防止壁流角度出发,应选取直径较小的水滴。(2)对无填料冷却塔的传热传质进行理论研究,建立基于Merkel假设的一维无填料冷却塔数值模型和神经网络模型,将传热传质模型与神经网络模型的计算同实验结果比较,发现神经网络结果更为接近实验数据。采用基于BP神经网络的非线性数学模型可以提高无填料冷却塔出口水温的计算精度,能够对无填料冷却塔的运行特性进行精确评价,本文所建BP神经网络模型对工程领域中冷却塔运行特性的评估过程具有重要参考意义。在对塔特性进行分析的基础上,对对喷式无填料冷却塔进行实验研究,并将计算结果与实验结果进行了比较。(3)为了更深入地研究对喷式无填料冷却塔的冷却特性,根据最新的碰撞分离模型,对水滴的对碰破碎情况进行研究,发现液滴碰撞现象很大程度上依赖液滴的属性。比较淡水与海水水滴的碰撞情况,发现由摩擦分离和反溅分离生成的海水水滴直径大于淡水水滴直径。根据碰撞模型对对喷式无填料冷却塔海水水滴冷却性能进行研究,发现在塔内将海水雾化后得到的水滴直径较小时,海水冷却效果可以与淡水冷却效果相近,为海水无填料冷却塔的研究奠定了理论基础。(4)在摒弃模型1两个假设的基础上,建立无填料冷却塔模型2,并对无填料冷却塔中过热空气与水的传热传质过程进行模拟,将两个模型的计算结果与实验结果进行比较,发现模型2的计算结果与实际测量结果更为接近,因模型2中未使用冷却塔计算中常用的假设条件,更接近冷却塔的实际运行状况,因此更为精确严格。使用模型1和模型2预测的出口水温差别不大。但在温度较高和湿度较小的条件下,模型1和模型2计算得到的出口水温差别增大,模型2预测的出口水温低于模型1预测的结果。这是由于模型1和模型2预测的出口空气温度存在差异。不管传统冷却塔中是否使用假设,两种方法预测的出口水温基本一致。因为在填料性能分析时传统冷却塔使用了实验测试的方法,减小了假设条件引起的差异。使用模型2预测的空气出口温度高于模型1的预测结果。周围温度较低时,两种方法预测的空气出口温度之间差异最小。若周围空气温度上升,空气湿度很小,两种模型预测的空气出口温度差异会增大。温度不变,空气湿度增加时,这种差异会减小。(5)利用　分析研究无填料冷却塔的性能,为对无填料冷却塔性能的深入研究提供了一个新方法。结果发现,水　从塔顶到塔底不断减少,水提供的　数量大于空气吸收的数量,因为系统产生了熵。为描述水和空气之间的可用　,沿着塔内每种工作流体给出　。通过研究发现,空气　主要由蒸发换热获得。　损自塔顶部至底部逐渐增大,这种分布特性可作为优化方法增大无填料冷却塔的运行性能。随着水滴的下落,到达塔底部时,水滴速度迅速增大,与新鲜空气之间的传热传质过程及其短暂,使得水传递给空气的有用能减少,　损增大。这是制约无填料冷却塔性能的一个重要因素。无填料冷却塔内由于没有填料整流作用,上部空气速度更大,这也是造成顶部　损小于底部　损的一个原因。(6)对引气雾化喷头进行试验研究,发现引气雾化喷头的流量系数平均为0.463,1mm以下的粒径达到85%以上。对同一喷头出口压力,1mm以下粒径所占的百分比沿径向从内到外逐渐降低,1mm以上粒径所占百分比逐渐升高。随着喷头出口压力增加,在径向同一位置粒径呈现逐步细化的趋势。喷头布水均匀性较好,所需工作压力低,雾化效果明显,适合于无填料冷却塔的使用。本文选用高效引气喷雾喷头设计了无填料上喷式雾化冷却塔。将喷雾装置安装在进风口上沿且均匀布置,水在塔内的流向与轴流风机抽吸的冷风同向,冷却水与空气接触有顺流、逆流和悬流三个过程,冷却也有顺流冷却,逆流冷却和悬流冷却三个过程,取代了填料塔的填料和布水装置,使整塔几乎成为一个空塔,结构简单,流动阻力明显减小。更多还原

【Abstract】 In view of the disadvantages of the conventional packed cooling tower (PCT), the SCT (Shower Cooling Tower) broke the designing way of the conventional cooling tower by doing away with the fills of the original countercurrent type of cooling tower and ameliorating the spray distribution of water. Eliminating the fill makes the tower fully empty which is of far-reaching significance in circulating water with high temperature, high turbidity. The research included the moving principles of hot water droplet in the cool air in the SCT and the heat and mass transfer between the droplet and the air based on the numerical model, the exergy analysis based on the second thermodynamic law, the experimental research guided by the theoretical research and through combining the head-on spray method, salt water cooling in the SCT is analysed, and the upward-spray SCT is designed. The research provided a theoretical basis for the application of the SCT. The research contents and conclusions are as follows:(1) Based on the force and the moving research of the droplet, the moving equations of the droplet in the SCT was derived and solved in detail. So the height of an SCT, the fitting location of the nozzles and the distance between the nozzles and the wall of the tower can be obtained in the designing of an SCT, which brings theoretical basis for the SCT designing. According to the different way of the water droplet and the air current, the water droplet flying process in the SCT was analyzed in detail, and the maximum and minimum radii of the possible existent water droplet under the different wind velocity were obtained. The influence of the equivalent diameter and the initial velocity of the water droplets on the moving up height was analysed. In order to investigate the coupled heat and mass transfer process between the droplet and the air, the evaporative model of water droplet was studied; and the conclusion that the evaporative mass of a droplet will not exceed 4%, so the decrease of its diameter after evaporation will not exceed 1%. So that the diameter of water droplet can be assumed to be constant, which will decrease calculated quantity greatly. Through the investigation of horizontal moving of the droplet, we found that as the diameter of the droplet is smaller, the horizontal moving distance is smaller. So in view of preventing the water sprayed on the wall, the smaller droplet is better.(2) Theoretical and experimental research on the heat and mass transfer of the SCT was conducted. A one-dimensional heat and masss transfer (HMT) numerical model based on Merkel assumptions and an artificial neural network (ANN) model were built. And experimental study was conducted. The results of the HMT model and ANN model were compared with the experimental data. It is shown that the ANN model is more close to the cooling process in the SCT. So the nonlinear model based on backpropagate ANN can improve the calculated precision of the outlet water temperature of the SCT. But the ANN model can only obtain the result, it is impossible to analyse and optimize each parameter. So it does not work for the ANN model to comprehend the heat and mass transfer status. However, the BP neural neural networks is of great significance in evaluating the operating characteristics of the SCT. Based on the investigation of the characteristics of the tower, experimental research on the tower is conducted, and then the influencing factors are analysed and the numerical and experimental results are compared.(3) In order to investigate the cooling characteristics of the impinging SCT, the impinging status of droplets was analysed through new breakup model. It is shown that the impinging phenomena of water droplets rely greatly on the attribution of the droplets. Through the experimental data published before (Ashgriz&Poo, 1990), dissipation factors are relavent with the weber number. By combining the impinging status of the water and salt water, the salt water number caused by stretching breakup and reflexive breakup is smaller than the water, because the surface tension of salt water is greater than the water. Based on impinging breakup model, salt water cooling in an SCT was analysed, which brought theoretical basis for salt water shower cooling tower.(4) Without applying the three of Merkel assumptions in model 1, SCT model 2 was built, the heat and mass transfer between the supersaturated air and the water was simulated, and the computed results and the experimental results were compared. It is shown that the results applying model 2 is more close to the experimental results. When the temperature of the ambient air is higher and the humidity is smaller, the outlet water temperature evaluated by model 2 is lower than model 1. This is because there is great difference between the outlet air temperature evaluated by model 1 and model 2. As the temperature of the ambient air is lower, the difference of the outlet air temperature between the two methods is smaller. As the ambient air temperature increases, if the air humidity is small, the difference of the evaluated outlet air temperature between the two methods will increase. Because the curve of the constant enthalpy in the supersaturated area is nearly upright, for constant air enthalpy, the air temperature is nearly constant and the moist air and mist included in the saturated air has no influence. But in the unsaturated area, the constant enthalpy curve is not upright, so the diffrence between the two points has big difference. So when the outlet air temperature is unsaturated, the results precision by applying model 1 is worse than the status that the outlet air temperature is supersaturated.(5) Based on the heat and mass model, the characteristics of the SCT by applying exergy is analyzed, which bring a method to study the SCT deeply. It is shown that the water exergy is defined as the usable energy included in the water that is decreased from the top to the bottom. The exergy quantity that the water provides is greater than obsorbs by the air, the system produes entropy. In order to dipict the usable exergy between the air and the water, the exergy of each fluid through the tower is given. It is shown that the air exergy via evaporative heat transfer is prominent. The exergy destruction increases from the top to the bottom, the distribution characteristics of exergy destruction can be used to optimize the operating characteristics of the SCT. Combining this conclusion, an important factor restricting SCT’s characteristics is that as the droplet falling to the bottom, the water droplet velocity increases greatly, which means the heat and mass transfer process between the droplet and the air is short, so the usable energy from the water to the air decreases and the exergy destruction increases. Moreover, in SCTs the air velocity in upper tower is greater, which causes the exergy destruction on the top is less than the bottom.更多还原